Polarizing plates having a structure in which a protective film is laminated on one or both surfaces of a polarizer formed with a polyvinyl alcohol (hereinafter, referred to ‘PVA’)-based resin dyed with dichroic dye or iodine using an adhesive have been commonly used. Triacetyl cellulose (TAC)-based films have been normally used as a polarizing plate protective film in the art, however, such TAC films have a problem of being readily deformed in high temperature and high humidity environments. Accordingly, protective films made of various materials capable of replacing TAC films have been recently developed, and for example, a method of using polyethylene terephthalate (PET), a cycloolefin polymer (COP), and an acryl-based film either alone or as a mixture thereof has been proposed.
Herein, aqueous adhesives formed with an aqueous solution of a polyvinyl alcohol-based resin are normally used as an adhesive used to attach the polarizer and the protective film. However, aqueous adhesives have a problem in that the use is limited depending on the material of a film, since adhesive strength is weak when acryl-based films or COP films and the like are used as the protective film instead of TAC. In addition to the problem of adhesive strength defects depending on the materials, the aqueous adhesive also has problems in that curls are generated in a polarizing plate due to a drying process of the aqueous adhesive, and initial optical properties are degraded when materials of the protective film used on both surfaces of a PVA element are different. Moreover, a drying process is absolutely required when the aqueous adhesive is used, and differences in moisture permeability, heat expansion and the like occur in the drying process leading to a problem of a defect rate increase. As an alternative to solve the problems described above, methods of using non-aqueous adhesives instead of aqueous adhesives have been proposed.
Non-aqueous adhesives for a polarizing plate that have been proposed so far may be divided into radical-curable adhesives and cation-curable adhesives depending on the types of curing. Cation-curable adhesives have an advantage in that excellent adhesive strength is obtained for films made of various materials, but have many disadvantages in the manufacturing process due to a low curing speed and a low degree of curing. Radical-curable adhesives having an acryl-based compound including a hydrophilic functional group as a main component have been proposed in order to solve the problems of such cation-curable adhesives. However, in the case of the radical-curable adhesive having an acryl-based compound as a main component, the adhesive has a low glass transition temperature after radical curing, therefore, there has been a problem in that a polarizing plate prepared using this adhesive has poor heat resistance reliability.
In view of the above, technologies additionally adding hydrophilic and multifunctional monomers and mixing these with the acryl-based compound have been proposed in order to increase a glass transition temperature of an adhesive. However, when these monomers are additionally added, the glass transition temperature of the adhesive may be raised, however, water resistance, which is a fundamental property that a polarizing plate needs to have, is very poor, and this has been a fatal disadvantage in using this adhesive in a polarizing plate.
Accordingly, new radical-curable adhesives having excellent adhesive strength between a polarizer and a protective film, and having excellent water resistance and heat resistance have been required.